Is there a reason you're filing this now instead of waiting until 3.1b2 has been released? Some developers don't update until fairly close to the day of release. You should probably give them a few days after release to be sure.

So, the add-on is compatible with 3.1b2 but doesn't work on one platform? That sounds like a bug that should be filed with the extension developer. We typically use Firefox::Extension Compatibility to track extensions that are not at all compatible with the next release of Firefox (in this case, Firefox 3.1).

That line was a red herring... but it turns out that this was a bug in the latest unstable Flashblock (we were calling Components.utils.lookupMethod instead of Components.lookupMethod). I uploaded a fixed version of Flashblock unstable to mozdev, but it might be a while until all the mirrors get it (if you want to be sure, check the MD5sum - should be 326fa184dc8931ac52adb57c78a7c986).

Flashblock has no platform dependent code.
WFM with Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2a1pre) Gecko/20081210 Minefield/3.2a1pre.
I changed Components.lookupMethod to Components.utils.lookupMethod because I was getting 'Components.lookupMethod is deprecated" strict warnings in the Error Console.
If Components.utils is available in Win32 and OSX but not ubuntu/linux this points to a packaging problem or something else

Lorenzo if you go to the Error Console and evaluate this:
"lookupMethod" in Components.utils
What do you get?
If I type in the Error Console:
Components.utils.lookupMethod
I get:
function lookupMethod() {
[native code]
}

OK. I can now reproduce the problem in 3.2a1pre (and probably in 3.1b2 as well, I just needed a bigger range of test pages to trigger this bug).
The Error Console message is:
Error: Permission denied for <http://www.bakketun.net> to get property XPCComponents.utils
Source file: chrome://flashblock/content/flashblock.xml
Line: 17

Perhaps you needed a bigger range of test pages because one of the lookupMethod calls was Components.lookupMethod and the other was Components.utils.lookupMethod? My change set them both to Components.lookupMethod.